The question of whether a cactus is a succulent is a common point of confusion for plant enthusiasts. The simple answer is that every cactus is a succulent, but the reverse is not true. This relationship is a matter of botanical classification, where the term “succulent” describes a functional adaptation, while “cactus” refers to a specific plant family. This distinction highlights the biological adaptations plants employ to survive in harsh, arid environments.
The Defining Trait of a Succulent
A plant is classified as a succulent based on its ability to store water in specialized, fleshy tissues. This adaptation allows these plants to thrive in environments with infrequent or unpredictable rainfall, such as deserts and semi-arid regions. Water is typically stored in the leaves, stems, or roots, giving the plants a thick, engorged appearance.
To conserve this stored moisture, most succulents have a thick, waxy outer layer, known as the cuticle. They also possess fewer stomata, which are the pores used for gas exchange.
Many succulents also employ a specialized form of photosynthesis called Crassulacean Acid Metabolism (CAM). This metabolic pathway allows the plant to open its stomata only at night when temperatures are cooler, taking in carbon dioxide and storing it as malic acid. During the day, the stomata remain closed to prevent water loss, and the stored carbon dioxide is released internally for photosynthesis.
The Unique Characteristics of Cacti
While all cacti are succulents, they belong exclusively to the plant family Cactaceae, distinguished by one singular feature: the areole. The areole is a specialized, cushion-like structure that appears as a small bump or fuzzy patch on the surface of the stem.
The areole is essentially a condensed, highly reduced branch, and it is the only place from which spines, hair, flowers, and new segments can grow on a cactus. Spines, which are modified leaves, emerge from the areole and serve as the plant’s primary defense and shading mechanism.
Other succulents may have spines or thorns, but those structures grow directly from the stem tissue, not from a specialized growth center like the areole. If a plant lacks areoles, regardless of its water-storing capacity, it cannot be classified as a member of the Cactaceae family.
Examples of Succulents That Are Not Cacti
The vast majority of succulents do not belong to the Cactaceae family, demonstrating the broad nature of the succulent classification. These plants come from many different plant families, including Crassulaceae, Agavaceae, and Euphorbiaceae. The primary difference is the lack of the defining areole structure, even though they have evolved similar water-storage solutions.
Leaf Succulents (Aloe, Echeveria, Sedum)
Plants in the genus Aloe, for instance, store their water entirely within their thick, fleshy leaves, which are typically arranged in a rosette pattern. The leaves of Echeveria and Sedum species, members of the Crassulaceae family, also serve as the main water-storage organs. These plants conserve water through leaf succulence rather than the stem succulence characteristic of most cacti.
Stem Succulents (Euphorbia)
Another large group of non-cactus succulents is the Euphorbia genus, which includes species like the African Milk Tree (Euphorbia trigona). Many euphorbias look remarkably similar to cacti, possessing thick, ribbed, and sometimes spiny stems, a phenomenon known as convergent evolution. However, their spines are modified stipules, not highly reduced leaves, and they do not grow from areoles, immediately excluding them from the Cactaceae family.